Underground car parks and underpasses are very, very expensive to build, if you are building a LOT of them.
Flyovers are also hideous and nobody wants to live near them.
Did you not see the current HS2 fiasco? They couldn’t even get people to accept a railway line going anywhere near their house (which is a large reason why the cost ballooned out of control). Good luck with trying to persuade people to accept enormous concrete flyovers in their areas!
Basically, if the UK wants to prosper, it has two ways it can do this.
It can (Option 1) insist on continuing with the car-centric model, and build the amount of car parking and road capacity needed to do this. This will massive amounts of sprawl and a huge increase in the size of cities like Oxford, which will involve eating heavily into the greenbelt, concreting over a load of countryside, and changing the appearance of town centers in ways that won’t be pretty - lots of pretty and historical buildings and streets would have to go, for example. Think of the kind of development they did in the 1960s, to see the sort of stuff I am talking about. But yes, you can prosper (in financial terms) by doing this. Look at American cities like Houston, actually most American cities - they have become financially prosperous via the car-centered model.
OR (Option 2) it can choose to densify. Get more people living in the urban core/center of towns in apartments (with a high % having no car), invest in good rail and tram links reaching out into the suburbs (where a high % of residents will manage with just one car per households). Make city centers very hard to drive into, and prioritize rail links for travel between cities. If you do this, you can prosper too (look at Tokyo and all the many European cities that have done this, and to a large extent London) and you also get to keep your pretty historical buildings and streets and maintain your lovely countryside. The downside is that everyone’s going to have to be grown-up about getting out of their cars a lot more. Society will also need to accept a higher % of people living in apartments rather than houses.
The trouble with the UK outside London is that cities at the moment are not actually doing either of these two options properly, in large part because most Brits simply won’t make the tough choices necessary or accept the inevitable trade-offs involved.
Brits, for the most part, go nuts when you try to build anything - even a railway line, let alone a bloody big motorway or flyover (which are far uglier and far more disruptive). Absolutely everything in this country, every type of building project, gets blocked, left right and center. Building on the greenbelt, even on ugly bits of it that have concrete all over them already, is intensely unpopular. NIMBYism is an issue in every country, but the UK does appear to have a particularly bad case of it.
But on the other hand, large numbers of Brits are also intensely resistant to the idea of getting out of their cars, walking more, using more public transit, or accepting the idea that a certain percentage of the population (*) might enjoy living in city center apartments. So many people here are intensely, emotionally attached to their cars, and get incredibly angry when anything else is suggested as a possibility.
And so the UK remains in a state of poor productivity and decline, with most areas outside London being effectively dependent on fiscal transfers from London. Because nobody wants to make any hard choices or accept any trade offs.
(*) Take Edgware, in London. A developer is trying to build dense apartment blocks on the site currently occupied by an underused, rundown shopping center and some ugly car parking land - all this is right on a good station, so the residents can live without cars or with minimal car use. Local residents (mostly over 60s) are up in arms, trying to block the development. I’d like to stress, nobody is forcing THEM to live in an apartment - their suburban detached houses are going to stay exactly as they are. But for the local residents, the mere idea of SOMEONE ELSE living in apartments in their neighborhoods is enough to provoke fear and anger. And people are raging about having the amount of car parking space reduced. Yet if the UK is to density and develop proper public transit, there is no way to do this without building more dense housing clustered around public transport hubs.